Hydraulic systems are employed in many circumstances to provide hydraulic power from a hydraulic power source to multiple loads. In particular, such hydraulic systems are commonly employed in a variety of work vehicles such as excavators and loader-backhoes. In such vehicles, the loads powered by the hydraulic systems may include a variety of hydraulically actuated devices such as piston-cylinder assemblies that lower, raise and rotate arms, and lower and raise buckets, as well as hydraulically-powered motors that drive tracks or wheels of the vehicles. Although the various hydraulically actuated devices typically are powered by a single source (e.g., a single pump), the rates of fluid flow to the different devices typically are independently controllable, through the use of separate control valves (typically spool valves) that are independently controlled by an operator of the work vehicle.
The operation of the hydraulically actuated devices depends upon the hydraulic fluid flow to those devices, which in turn depends upon the cross-sectional areas of metering orifices of the control valves between the pressure source and the hydraulically actuated devices, and also upon the pressure differentials across those metering orifices.
To facilitate control, hydraulic systems often are pressure compensated, that is, designed to set and maintain the pressure differentials across the metering orifices of the control valves, so that controlling of the valves by an operator only tends to vary the cross-sectional areas of the orifices of those valves but not the pressure differentials across those orifices. Such pressure compensated hydraulic systems typically include pressure compensation valves positioned between the respective control valves and the respective hydraulically actuated devices. The pressure compensation valves control the pressures existing on the downstream sides of the metering orifices to produce the desired pressure differentials across the metering orifices.
Such pressure-compensated hydraulic systems normally ensure that the same particular pressure differential (e.g., a pump margin pressure) occurs across each of the control valves. Nevertheless, it may be desirable in some hydraulic systems to have a lower pressure differential across selected valves to reduce the hydraulic fluid flow through those valves. For example, in the case of an excavator, it may be desirable to provide normal hydraulic fluid flow to the cylinders that control lifting or other movement of an arm or bucket of the excavator, or to accessories of the excavator such as a trenching device, yet at the same time desirable to provide reduced hydraulic fluid flow to the hydraulic motors controlling the speeds of the tracks of the excavator so that the excavator travels at reduced speeds. Therefore, there is a need in some hydraulic systems to provide a pressure differential across metering orifices in selected control valves which is less than the pressure differential across other control valves.
This capability of providing adjustable control of the pressure differentials across multiple control valves in an even manner is desirable in many circumstances, since it is often desirable that multiple hydraulic devices of a hydraulic system should receive precisely identical amounts of hydraulic fluid flow when an operator sets the respective control valves identically. For example, with respect to the excavator discussed above, it would be desirable that the hydraulic motors corresponding to the left and right tracks of the excavator be driven at the exact same speed assuming that the operator of the excavator set the control valves for those motors to the same level.
U.S. Pat. No. 6,895,852 discloses an apparatus having a valve assembly with pressure compensated valve sections. The apparatus includes an adjustable pressure reducing valve that communicates pressure from a source (e.g., a pump) to the particular compensation valves that are coupled to the control valves for which adjustable control is desired. The opposing actuation ports of the adjustable pressure reducing valve are coupled, respectively, to the pressure applied to those particular compensation valves and to the highest load pressure plus an adjustment spring pressure. Consequently, the pressure applied to the particular compensation valves exceeds that of the highest load pressure by the adjustment spring pressure, which results in reduced pressure differentials across the control valves associated with those compensation valves. Because the adjustable pressure reducing valve is in communication with each of the particular compensation valves that are coupled to the control valves for which adjustable control is desired, and because the single adjustment spring pressure determines the operation of that adjustable pressure reducing valve, an operator only needs to make a single adjustment to the single adjustment spring pressure to produce the same changes to the pressure differentials across each of the control valves for which adjustable control is desired. Also disclosed is the use of another valve that is coupled between the adjustable pressure reducing valve, the highest load pressure and the particular compensation valves of interest. The reduction in the pressure differentials produced by the adjustable pressure reducing valve can be switched on and off by alternatively coupling the particular compensation valves to the output of the adjustable pressure reducing valve and to the highest load pressure, respectively.